Flexible and stretchable sensors formed by patterned spalling

ABSTRACT

A material removal process referred to as spalling is used to provide flexible and stretchable sensors that can be used for healthcare monitoring, bio-medical devices, wearable electronic devices, artificial skin, large area sensing, etc. The flexible and stretchable sensors of the present application have high sensitivity that is comparable to that of a bulk silicon sensor. The flexible and stretchable sensors comprise single crystalline spring-like structures that couple various resistor structures together.

BACKGROUND

The present application relates to semiconductor technology. Moreparticularly, the present application relates to a high sensitivitysensor that is flexible and stretchable as well as a method of formingthe same.

Sensors that are flexible and stretchable have important applications intoday's society and have been used, for example, in healthcaremonitoring, bio-medical devices, wearable electronic devices, artificialskin, and large area sensing. Prior art sensors use either singlecrystalline bulk devices or thin film amorphous/polycrystallinematerials. Single crystalline sensors usually have high sensitivity, butare not flexible or stretchable. Thin film sensors are flexible andstretchable, but are not sensitive. Thus, there is a need for providingsensors that have a combination of high sensitivity, flexibility andstretchability.

SUMMARY

A material removal process referred to as spalling is used to provideflexible and stretchable sensors that can be used for healthcaremonitoring, bio-medical devices, wearable electronic devices, artificialskin, large area sensing, etc. The flexible and stretchable sensors ofthe present application have high sensitivity that is comparable to thatof a bulk silicon sensor and with a piezoresistive gauge factor ofgreater than 60. The flexible and stretchable sensors comprise singlecrystalline spring-like structures that couple various resistorstructures together. That is, the single crystalline spring-likestructures serves as wires within the sensor of the present application.By “spring-like” it is meant that the single crystalline structure iscapable of resuming its' original shape after stretching or compression.

In one aspect of the present application, a method of forming a flexibleand stretchable sensor is provided. In one embodiment of the presentapplication, the method includes providing a single crystalline materialcontaining base substrate having a plurality of resistor structuresembedded within the single crystalline material containing basesubstrate. A portion of the single crystalline material containing basesubstrate and each resistor structure are then removed from the singlecrystalline material containing base substrate by spalling. Spallingprovides a spalled structure including the plurality of resistorstructures, wherein each neighboring pair of resistor structures isinterconnected by a single crystalline material portion of the basesubstrate. A flexible substrate is then formed on an exposed surface ofthe spalled structure.

In another aspect of the present application, a structure is providedthat includes a flexible and stretchable sensor embedded within aflexible substrate. The flexible and stretchable sensor of the presentapplication includes a single crystalline spring-like structure thatcouples a neighboring pair of resistor structures together.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplary structure including asingle crystalline material containing base substrate having a pluralityof resistor structures embedded within the single crystalline materialcontaining base substrate in accordance with an embodiment of thepresent application.

FIG. 2 is a cross sectional view of the exemplary structure of FIG. 1after forming a first photoresist structure having a first opening thatexposes a topmost surface of each resistor structure.

FIG. 3 is a cross sectional view of the exemplary structure of FIG. 2after forming a plating seed layer on the exposed surfaces of the firstphotoresist structure and the exposed topmost surface of each resistorstructure.

FIG. 4 is a cross sectional view of the exemplary structure of FIG. 3after forming a second photoresist structure on portions of the platingseed layer, wherein the second photoresist structure has a secondopening that coincides within the first opening.

FIG. 5 is a cross sectional view of the exemplary structure of FIG. 4after forming a metal stressor structure within the second opening.

FIG. 6 is a cross sectional view of the exemplary structure of FIG. 5after forming a handle substrate on a topmost surface of the secondphotoresist structure and on exposed surfaces of the metal stressorstructure.

FIG. 7 is a cross sectional view of the exemplary structure of FIG. 6after performing spalling to provide a spalled structure containing theplurality of resistor structures and a portion of the single crystallinematerial containing base substrate.

FIG. 8 is a cross sectional view of the exemplary structure of FIG. 7after forming a flexible substrate.

FIG. 9 is a cross sectional view of the exemplary structure of FIG. 8after removing an entirety of the handle substrate, the secondphotoresist structure and the first photoresist structure, and a portionof the plating seed layer from the spalled structure.

FIG. 10 is a cross sectional view of the exemplary structure of FIG. 9after removing the remaining plating seed layer and the metal stressorstructure from the spalled structure.

FIG. 11 is an actual photograph of one example of the exemplarystructure of FIG. 1.

FIG. 12 is an actual photograph of one example of the exemplarystructure of FIG. 7.

FIG. 13 is an actual photograph of one example of the exemplarystructure of FIG. 10.

FIG. 14 is an actual photograph of the exemplary structure of FIG. 13during a bending test.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

As mentioned above, a material removal process referred to as spallingis used to provide flexible and stretchable sensors that can be used forhealthcare monitoring, bio-medical devices, wearable electronic devices,artificial skin, large area sensing, etc. The flexible and stretchablesensors of the present application have high sensitivity that iscomparable to that of a bulk silicon sensor and with a piezoresistivegauge factor of greater than 60. The flexible and stretchable sensorscomprise single crystalline spring-like structures that couple variousresistor structures together. That is, the single crystallinespring-like structures serves as wires within the sensor of the presentapplication. The bending radius of the sensors of the presentapplication can be smaller than 1 cm, and the stretchable range is about20%.

Referring now to FIG. 1, there is illustrated a cross sectional view ofan exemplary structure including a single crystalline materialcontaining base substrate 10 having a plurality of resistor structures(one of which is shown in FIG. 1 as element 12) embedded within thesingle crystalline material containing base substrate 10 in accordancewith an embodiment of the present application. The other resistorstructures would be located in front of and/or behind and/or to eitherside of the resistor structure 12 shown in FIG. 1. A portion of thesingle crystalline material containing base substrate 10 surrounds eachsidewall surface and a bottom surface of each resistor structure 12; thetopmost surface of each resistor structure is exposed. The term “singlecrystalline” is used throughout the present application to denote amaterial in which the crystal lattice of the entire sample is continuousand unbroken to the edges of the sample, with no grain boundaries.

The single crystalline material containing base substrate 10 that isemployed in the present application includes a material or stack ofmaterials whose fracture toughness is less than that of a stressinducing metal-containing material to be subsequently formed. Fracturetoughness is a property which describes the ability of a materialcontaining a crack to resist fracture. Fracture toughness is denotedK_(Ic). The subscript Ic denotes mode I crack opening under a normaltensile stress perpendicular to the crack, and c signifies that it is acritical value. Mode I fracture toughness is typically the mostimportant value because spalling mode fracture usually occurs at alocation in the substrate where mode II stress (shearing) is zero.Fracture toughness is a quantitative way of expressing a material'sresistance to brittle fracture when a crack is present.

In some embodiments, the material or stack of materials that providesthe single crystalline material containing base substrate 10 has a highpiezoelectric coefficient. By “high piezoelectric coefficient” it ismeant a piezoelectric coefficient of from 2E⁻¹² m/V or greater.

In one example, the single crystalline material containing basesubstrate 10 may include a semiconductor material such as, for example,Si, Ge, SiGe, SiGeC, SiC, or a compound semiconductors such as, forexample, III-V compound semiconductors or II-VI compound semiconductors.In some embodiments, the single crystalline material containing basesubstrate 10 may include a single semiconductor material. In otherembodiments, a multilayered semiconductor material stack containing atleast two different semiconductor materials can be used as the singlecrystalline material containing base substrate 10. In some embodiments,the single crystalline material containing base substrate 10 is a bulksemiconductor material (i.e., the substrate is composed entirely of atleast one semiconductor material). In other embodiments, the singlecrystalline material containing base substrate 10 may comprise a layeredsemiconductor material such as, for example, asemiconductor-on-insulator or a semiconductor on a polymeric substrate.Illustrated examples of semiconductor-on-insulator substrates that canbe employed as single crystalline material containing base substrate 10include silicon-on-insulators and silicon-germanium-on-insulators.

In one embodiment of the present application, each resistor structure 12is entirely embedded within the base substrate 10. In anotherembodiment, each resistor structure 12 can have a lower portion embeddedwithin the single crystalline material containing base substrate 10 andan upper portion that extends above a topmost surface of the singlecrystalline material containing base substrate 10. In yet otherembodiments, a first set of the resistor structures 12 can be entirelyembedded within the single crystalline material containing basesubstrate 10, while a second set of resistor structures 12 can bepartially embedded within the single crystalline material containingbase substrate 10.

In one embodiment of the present application (and as shown), theresistor structures 12 can be a semiconductor material that is dopedwith an n-type or a p-type dopant (i.e., doped semiconductor material).The term “p-type” refers to the addition of impurities to an intrinsicsemiconductor material that creates deficiencies of valence electrons.In a silicon-containing semiconductor material, examples of p-typedopants, i.e., impurities, include, but are not limited to, boron,aluminum, gallium and indium. “N-type” refers to the addition ofimpurities that contributes free electrons to an intrinsicsemiconductor. In a silicon containing semiconductor material, examplesof n-type dopants, i.e., impurities, include, but are not limited to,antimony, arsenic and phosphorous. The doped semiconductor material canhave a dopant concentration that can be within a range from 1E16atoms/cm³ to 1E19 atoms/cm³. The semiconductor material that is dopedwith the n-type or the p-type dopant can include one of thesemiconductor materials mentioned above for the single crystallinematerial containing base substrate 10. In one embodiment, thesemiconductor material that is doped with the n-type or p-type dopantcan be a same semiconductor material as the single crystalline materialcontaining base substrate 10. In another embodiment, the semiconductormaterial that is doped with the n-type or the p-type dopant can be adifferent semiconductor material than the single crystalline materialcontaining base substrate 10.

In some embodiments, the semiconductor material that is doped with then-type or the p-type dopant can be formed by introducing the dopantwithin predetermined portions of the single crystalline materialcontaining base substrate 10. In such an embodiment, ion implantation orgas phase doping may be used to introduce the dopant within the singlecrystalline material containing base substrate 10. In other embodiments,a trench can be formed into the single crystalline material containingbase substrate 10 by lithography and etching and thereafter the resistorstructure 12 can be formed by utilizing an epitaxial growth (ordeposition) process. In such an embodiment, the dopant can be introducedduring the epitaxial growth process or after epitaxial growth utilizingion implantation or gas phase doping.

In some embodiments, the resistor structure 12 may include doped (n-typeor p-type) polysilicon, a ceramic, a carbon film, a metal oxide, or anyanother material or combination of materials that can function as aresistor. A resistor is a passive two terminal electrical component thatimplements electrical resistance as a circuit element. Resistors act toreduce current flow, and, at the same time, act to lower voltage levelswithin circuits. In the present application, the resistor structures 12can have any design and can be used within a sensor that can be used forhealthcare monitoring, bio-medical devices, wearable electronic devices,artificial skin, large area sensing, etc.

Referring now to FIG. 2, there is illustrated the exemplary structure ofFIG. 1 after forming a first photoresist structure 14 having a firstopening 15 that exposes a topmost surface of each resistor structure 12.The first photoresist structure 14 includes a photoresist material suchas, for example, a positive-tone photoresist composition, anegative-tone photoresist composition or a hybrid-tone photoresistcomposition. The photoresist material may be formed by a depositionprocess such as, for example, spin-on coating. After forming thephotoresist material, the deposited photoresist material is subjected toa pattern of irradiation. Next, the exposed photoresist material isdeveloped utilizing a conventional resist developer. This provides thefirst photoresist structure 14 having the first opening 15 mentionedabove.

Referring now to FIG. 3, there is illustrated the exemplary structure ofFIG. 2 after forming a plating seed layer 16 on the exposed surfaces ofthe first photoresist structure 14 and an exposed topmost surface of theresistor structures 12.

The plating seed layer 16 is employed to selectively promote subsequentplating of a preselected stress inducing metal-containing material. Theplating seed layer 16 may include, for example, a single layer of Ni ora layered structure of two or more metals such as Ti/Ni, Ti/Ag, Ti/Au,Cr/Ni, Cr/Ag, Cr/Au, Al(bottom)/Ti/Ni(top), etc. The plating seed layer16 is a contiguous layer (i.e., a layer without any interruptions orbreaks). In one embodiment of the present application, the plating seedlayer 16 may have thickness from 2 nm to 1 micron. Other thicknessesthat are lesser than, or greater than this thickness range may also beemployed as the thickness of the plating seed layer 16.

The plating seed layer 16 can be formed by a conventional depositionprocess including, for example, chemical vapor deposition (CVD),plasma-enhanced chemical vapor deposition (PECVD), atomic layerdeposition (ALD), or physical vapor deposition (PVD) techniques that mayinclude evaporation and/or sputtering. In accordance with the presentapplication, the plating seed layer 16 is formed at a temperature whichdoes not effectuate spontaneous spalling to occur within the singlecrystalline material containing base substrate 10.

In some embodiments, and after the forming the plating seed layer 16, ametal-containing adhesion layer (not independently shown) can be formed.Although not specifically shown, the metal-containing adhesion layer hasthe same basic form as the plating seed layer 16 shown in FIG. 3; assuch, element 16 may represent a stack of, from bottom to top, theplating seed layer and the metal-containing adhesion layer.

The optional metal-containing adhesion layer that can be employed in thepresent application includes any metal adhesion material such as, butnot limited to, Ti/W, Ti, Cr, Ni or any combination thereof. Theoptional metal-containing adhesion layer may comprise a single layer orit may include a multilayered structure comprising at least two layersof different metal adhesion materials.

When present, the optional metal-containing adhesion layer is formed ata temperature which does not effectuate spontaneous spalling to occurwithin the single crystalline material containing base substrate 10. Inone embodiment, the optional metal-containing adhesion layer can beformed at a temperature from 15° C. to 180° C. The metal-containingadhesion layer, which may be optionally employed, can be formedutilizing deposition techniques that are well known to those skilled inthe art. For example, the optional metal-containing adhesion layer canbe formed by sputtering, chemical vapor deposition, plasma enhancedchemical vapor deposition, chemical solution deposition, physical vapordeposition, or plating. When sputter deposition is employed, the sputterdeposition process may further include an in-situ sputter clean processbefore the deposition.

When employed, the optional metal-containing adhesion layer can have athickness from 5 nm to 300 nm, although other thickness that are lesserthan, or greater than, the aforementioned thickness range may also beemployed in the present application.

Referring now to FIG. 4, there is illustrated the exemplary structure ofFIG. 3 after forming a second photoresist structure 18 on portions ofthe plating seed layer 16, wherein the second photoresist structure 18has a second opening 19 that coincides within the first opening 15. Thesecond photoresist structure 18 may include one of the photoresistmaterials mentioned above for the first photoresist structure 14. In oneembodiment of the present application, the first and second photoresiststructures (14, 18) are composed of a same photoresist composition. Inanother embodiment of the present application, the first photoresiststructure 14 is composed of a different photoresist composition than thesecond photoresist structure 18. The second photoresist structure 18 canbe formed utilizing the technique mentioned above in forming the firstphotoresist structure 14.

Referring now to FIG. 5, there is illustrated the exemplary structure ofFIG. 4 after forming a metal stressor structure 20 within the secondopening 19. In one embodiment and as is shown, a bottommost surface ofthe metal stressor structure 20 is in direct contact with an exposedsurface of the plating seed layer 16. In another embodiment (not shown),the bottommost surface of the metal stressor structure 20 is in directcontact with an exposed surface of the metal-containing adhesion layer,which is formed atop the plating seed layer. The metal stressorstructure 20 has a topmost surface that can be coplanar with (not shown)or slightly above the topmost surface of the second photoresiststructure 18.

The metal stressor structure 20 is composed of a stress inducingmetal-containing material. In accordance with the present application,the metal stressor structure 20 has a critical thickness and a stressvalue that cause spalling mode fracture to occur within the singlecrystalline material containing base substrate 10. In particular, metalstressor structure 20 has a critical thickness in which spalling isinitiated somewhere in between the topmost and bottommost surface of thesingle crystalline material containing base substrate 10. By ‘critical’,it is meant that for a given stress inducing material and singlecrystalline material containing base substrate material combination, athickness value and a stressor value for the metal stressor structure 20is chosen that render spalling mode fracture possible (can produce aK_(I) value greater than the K_(IC) of the substrate). In someembodiments, the stress value can be adjusted by tuning the depositionconditions of the stress inducing metal-containing material thatprovides the metal stressor structure 20. For example, in the case ofsputter deposition of stress inducing metal-containing material thatprovides the metal stressor structure 20, the gas pressure can be usedto tune the stress value as described in Thorton and Hoffman, J. Vac.Sci. Technol., 14 (1977) p. 164.

The thickness of the metal stressor structure 20 is chosen to providethe desired fracture depth somewhere within the single crystallinematerial containing base substrate 10. For example, if the stressinducing material that provides the metal stressor structure 20 ischosen to be Ni, then fracture will occur at a depth below the metalstressor structure 20 roughly 2 to 3 times the Ni thickness. The stressvalue for the stress inducing material that provides the metal stressorstructure 20 is then chosen to satisfy the critical condition forspalling mode fracture. This can be estimated by inverting the empiricalequation given by t*=[(2.5×10⁶)(K_(IC) ^(3/2))]/σ², where t* is thecritical stressor layer thickness (in microns), K_(IC) is the fracturetoughness (in units of MPa·m^(1/2)) of the single crystalline materialcontaining base substrate 10 and σ is the stress value of the stressinducing material that provides the metal stressor structure 20 (in MPaor megapascals). The above expression is a guide, in practice, spallingcan occur at stress or thickness values up to 20% less than thatpredicted by the above expression.

The stress inducing metal-containing material that can provide the metalstressor structure 20 may include, for example, Ni, Cr, Fe, Mo, Ti or W.Alloys of these metals can also be employed. In one embodiment, theinducing metal-containing material that provides the metal stressorstructure 20 includes at least one layer consisting of Ni.

The stress inducing metal-containing material that can provide the metalstressor structure 20 can be formed utilizing a deposition process, suchas, for example, sputtering, chemical vapor deposition, plasma enhancedchemical vapor deposition, chemical solution deposition, physical vapordeposition, or plating. The deposition of the stress inducingmetal-containing material that can provide the metal stressor structure20 may be performed at a temperature from room temperature (15° C.-40°C.) to 60° C. Other deposition temperatures are possible so long as theselected deposition temperature does not cause spontaneous spalling ofthe single crystalline material containing base substrate 10. In someembodiments of the present application, the deposited stress inducingmetal-containing material can be patterned by lithography and etching toprovide the metal stressor structure 20 shown in FIG. 5.

The metal stressor structure 20 may have a thickness from 1 μm to 50 μm.Other thicknesses for the metal stressor structure 20 that are lesserthan, or greater than the aforementioned thickness range can also beemployed in the present application.

Referring now to FIG. 6, there is illustrated the exemplary structure ofFIG. 5 after forming a handle substrate 22 on a topmost surface of thesecond photoresist structure 18 and on exposed surfaces of the metalstressor structure 20. The handle substrate 22 of the presentapplication can include any flexible material which has a minimum radiusof curvature of less than 30 cm. Illustrative examples of flexiblematerials that can be employed as the handle substrate 22 include ametal foil, a polyimide foil or a tape.

The handle substrate 22 can be used to provide better fracture controland more versatility in handling the spalled portion of the singlecrystalline material containing base substrate 10. Moreover, the handlesubstrate 22 can be used to guide the crack propagation during thespalling process of the present application. The handle substrate 22 ofthe present application is typically, but not necessarily, formed at afirst temperature which is at room temperature (15° C.-40° C.).

When a tape is employed as the flexible material that provides thehandle substrate 22, the tape may include a pressure sensitive tape. By“pressure sensitive tape,” it is meant an adhesive tape that will stickwith application of pressure, without the need for solvent, heat, orwater for activation. Typically, the pressure sensitive tape that isemployed in the present application includes at least an adhesive layerand a base layer. Materials for the adhesive layer and the base layer ofthe pressure sensitive tape include polymeric materials such as, forexample, acrylics, polyesters, olefins, and vinyls, with or withoutsuitable plasticizers. Plasticizers are additives that can increase theplasticity of the polymeric material to which they are added. Someexamples of tapes that can be used in the present application as handlesubstrate 22 include, Nitto Denko 3193MS thermal release tape, KaptonKPT-1, and Diversified Biotech's CLEAR-170 (acrylic adhesive, vinylbase).

The handle substrate 22 can be formed utilizing deposition techniquesthat are well known to those skilled in the art including, for example,mechanical pressure, dip coating, spin-coating, brush coating,sputtering, chemical vapor deposition, plasma enhanced chemical vapordeposition, chemical solution deposition, physical vapor deposition, orplating. When a tape is employed as the handle substrate 22, the tapecan be applied by hand or by mechanical means to the structure. The tapecan be formed utilizing techniques well known in the art or they can becommercially purchased from any well known adhesive tape manufacturer.

In one embodiment of the present application, the handle substrate 22may have a thickness of from 5 μm to 500 μm. Other thicknesses for thehandle substrate 22 that are lesser than, or greater than, theaforementioned thickness range can also be employed in the presentapplication.

As shown, the handle substrate 22 typically has a length that extendsbeyond the length of the single crystalline material containing basesubstrate 10. As such, it is possible to process multiple singlecrystalline material containing substrates utilizing a single handlesubstrate 22.

Referring now to FIG. 7, there is illustrated the exemplary structure ofFIG. 6 after performing spalling to provide a spalled structure 50containing the resistor structures 12 and a portion of the singlecrystalline material containing base substrate 10; the spalled portionof the single crystalline material containing base substrate 10 can bereferred to herein as a single crystalline material portion 10P. Thespalling is a patterned removal process since only a preselected area ofthe single crystalline material containing base substrate 10 whichcontains the resistor structures 12 is removed. In addition to theresistor structures 12 and a portion of the single crystalline materialcontaining base substrate 10, the spalled structure 50 further includesthe handle substrate 22, the metal stressor structure 20, the secondphotoresist structure 18, the plating seed layer 16, the optionalmetal-containing adhesion layer, and the first photoresist structure 14.

Although not shown in the cross sectional view, a portion of the singlecrystalline material portion 10P is present between each resistorstructure 12. The portion of the single crystalline material portion 10Pthat is present between each resistor structure 12 is a singlecrystalline spring-like structure which connects the various resistorstructures 12. The single crystalline spring-like structure (labeled as10P) is shown clearly in FIG. 13.

The term “spalling” is used throughout the present application to denotea material removal process in which a stressor material induces crackformation and propagation within an underlying material whose fracturetoughness is less than the stressor layer. In one embodiment of thepresent application, spalling includes pulling or peeling the handlesubstrate 22 to remove the spalled structure 50 from the singlecrystalline material containing base substrate 10. In one embodiment,spalling can be initiated at room temperature (i.e., 15° C. to 40° C.).In other embodiments, spalling can be performed at a temperature from100° C. and below. In some embodiments of the present application,spalling can be initiated by lowering the temperature at a fixedcontinuous rate. By “fixed continuous rate” it is mean, for example, 20°C. per second utilizing an electronically controlled cooling table orchamber. This method of cooling allows one to reach a pre-specifiedtemperature at which user-defined spalling initiation can induce apre-determined spalling depth that may be different than that dictatedby mere structural parameters (i.e., stressor layer stress andthickness, and fracture toughness of substrate).

The thickness of the single crystalline material portion 10P that can beremoved from the single crystalline material containing base substrate10 varies depending on the material of the metal stressor structure 20and the material of the single crystalline material containing basesubstrate 10 itself. In one embodiment, the single crystalline materialportion 10P that is removed from the single crystalline materialcontaining base substrate 10 has a thickness of less than 100 microns.

Referring now to FIG. 8, there is illustrated the exemplary structure ofFIG. 7 after forming a flexible substrate 24 on bottommost surface ofthe spalled structure 50. The flexible substrate 24 may include apolyimide foil or a tape as mentioned above for handle substrate 22. Inone embodiment, the flexible substrate 24 and the handle substrate areboth composed of a tape. The flexible substrate 24 can be appliedutilizing one of the techniques mentioned above in applying the handlesubstrate 22 to the exemplary structure shown in FIG. 6.

The flexible substrate 24 has a radius of curvature within the rangementioned above for the handle substrate 22. Like the handle substrate22, the flexible substrate 24 has a length that extends beyond thelength of the base substrate 10. The length of the flexible substrate 24may be less than, equal to, or greater than the length of the handlesubstrate 22.

Referring now to FIG. 9, there is illustrated the exemplary structure ofFIG. 8 after removing an entirety of the handle substrate 22, the secondphotoresist structure 18 and the first photoresist structure 14, and aportion of the plating seed layer 16 from the spalled structure 50.

The handle substrate 22, the second photoresist structure 18 and thefirst photoresist structure 14, and a portion of the plating seed layer16 can be removed utilizing conventional techniques well known to thoseskilled in the art. For example, and in one embodiment, aqua regia(HNO₃/HCl) can be used for removing the handle substrate 22, the secondphotoresist structure 18 and the first photoresist structure 14, and aportion of the plating seed layer 16 from the spalled structure 50. Inanother example, UV or heat treatment is used to remove the handlesubstrate 22, a photoresist stripping process such as ashing can be usedto remove the second photoresist structure 18 followed by a chemicaletch to remove the portions of the plating seed layer 16, and/or themetal-containing adhesion layer, followed by a second photoresiststripping process to remove the first photoresist structure 14.

Referring now to FIG. 10, there is illustrated the exemplary structureof FIG. 9 after removing the remaining plating seed layer 16 and themetal stressor structure 20 from the spalled structure 50. The metalstressor structure 20 can be removed utilizing a first chemical etch,while another chemical etch can be used to remove the remaining platingseed layer 16 and any remaining metal-containing adhesion layer.

FIGS. 11-14 are actual photographs of one example of the exemplarystructure of the present application through various processing steps.Notably, FIG. 11 is an actual photograph of one example of the exemplarystructure of FIG. 1, FIG. 12 is an actual photograph of one example ofthe exemplary structure of FIG. 7, FIG. 13 is an actual photograph ofone example of the exemplary structure of FIG. 10, and FIG. 14 is anactual photograph of the exemplary structure of FIG. 13 during a bendingtest. The bending test used to provide the exemplary structure shown inFIG. 14 was performed on a cylindrical tube with a fixed radius.

Notably, FIGS. 10, 13 and 14 illustrate a structure in accordance withan embodiment of the present application, which includes a flexible andstretchable sensor embedded within a flexible substrate 24, the flexibleand stretchable sensor comprises a single crystalline spring-likestructure 10P that couples a neighboring pair of resistor structures 12together. Within each sensor there is a plurality of neighboring pair ofresistor structures 12 and a plurality of single crystalline spring-likestructures 10P. A surface of each resistor structure 12 is exposed.

While the present application has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A structure comprising: a flexible andstretchable sensor embedded within a flexible substrate, said flexibleand stretchable sensor comprising a single crystalline spring-likestructure coupling a neighboring pair of resistor structures together;and at least one other flexible and stretchable sensor embedded withinsaid flexible substrate, said at least one other flexible andstretchable sensor comprising another single crystalline spring-likestructure coupling another neighboring pair of resistor structurestogether.
 2. The structure of claim 1, wherein said single crystallinespring-like structure is a single crystalline semiconductor material. 3.The structure of claim 2, wherein said single crystalline semiconductormaterial is silicon.
 4. The structure of claim 1, wherein each resistorstructure comprises a doped semiconductor material.
 5. The structure ofclaim 4, wherein said doped semiconductor material and said singlecrystalline spring-like structure both comprise a same semiconductormaterial.
 6. The structure of claim 1, wherein said flexible substrateis a tape.
 7. The structure of claim 1, wherein a surface of eachresistor structure is exposed.
 8. The structure of claim 7, wherein saidsurface of each resistor is coplanar with a topmost surface of saidflexible substrate.
 9. The structure of claim 1, wherein singlecrystalline spring-like structure is located directly beneath eachresistor structure.
 10. The structure of claim 1, wherein said singlecrystalline spring-like structure has a piezoelectric coefficient offrom 2E¹² mV or greater.
 11. The structure of claim 2, wherein saidsingle crystalline semiconductor material is composed of Ge, SiGe,SiGeC, SiC, a III-V compound semiconductor or a II-VI compoundsemiconductor.
 12. The structure of claim 4, wherein said dopedsemiconductor material comprises a p-type semiconductor material. 13.The structure of claim 4, wherein said doped semiconductor materialcomprises an n-type semiconductor material.
 14. The structure of claim4, wherein aid doped semiconductor material has a dopant concentrationof from 1E16 atoms/cm³ to 1E19 atoms/cm³.
 15. The structure of claim 1,wherein each resistor comprises doped polysilicon.
 16. The structure ofclaim 1, wherein each resistor comprises a ceramic.
 17. The structure ofclaim 1, wherein each resistor comprises a carbon film.
 18. Thestructure of claim 1, wherein each resistor comprises a metal oxide.